Remove flatiron/union dependency

We need to get off the outdated flatiron/union dependency because it's no longer necessary because Streams--see https://github.com/flatiron/union/pull/61#issuecomment-443856321 for more.

@thornjad this relates to the PR you sent to flatiron/union. If you've still got coding cycle time, I'd love your help on this one!

PR's from anyone interested are also welcome.